Variable displacement vane pumps are well known and are used in a variety of systems. One use for such pumps which is becoming increasingly common is as lubrication oil pumps on internal combustion engines. Lubrication oil pumps in internal combustion engines operate over a wide range of speeds, as the engine operating speed changes, resulting in the output volume and the output pressure (as the output of these pumps is generally supplied to a lubrication system which can be approximately modeled as a fixed size orifice) of the pumps changing with their operating speed.
Generally, an internal combustion engine requires the lubrication oil pressure to increase with engine operating speed from a minimum necessary level at the lowest operating speed of the engine to a maximum desired pressure level, at a given higher operating speed of the engine. The engine's oil pressure requirements do not increase beyond the maximum desired pressure level at any other operating conditions.
As the maximum desired oil pressure level is output from the pump under normal temperature conditions at engine operating speeds well below the maximum engine operating speed, the lubrication oil pump will provide an oversupply of lubrication oil over a significant portion of the engine operating speed and temperature ranges unless its displacement is decreased once the maximum desired oil pressure has been reached. The oversupply of lubricating oil is undesired as it wastes energy, reducing fuel efficiency of the engine, and in some applications as the oversupply results in an overpressure which can damage the engine and/or other components of the engine system.
Accordingly, variable displacement vane pumps include a moveable control ring which allows the displacement capacity per revolution of the pump to be changed. Typically a control spring biases the control ring to the position of maximum displacement and a feedback mechanism, such as a control piston connected to a supply of pressurized oil from the pump, acts to move the control ring towards the position of minimum displacement as the operating speed of the pump increases in order to regulate oil pressure to a specified level.
At engine start up, the feedback mechanism cannot overcome the biasing force of the control spring and the control ring will be in the maximum displacement position to ensure that the pump supplies lubricating oil at the minimum necessary pressure. As the operating speed of the pump increases, the output pressure of the pump increases and the feedback mechanism begins to counter the biasing force of the control spring, reducing the displacement of the pump by moving the control ring towards the minimum displacement position and thus preventing undesired overpressure conditions in the output of the pump.
Ideally, once the output of the pump reaches the maximum desired pressure level for the engine (i.e. the regulated operating region), further increases in the operating speed of the engine and pump result in corresponding decreases in the displacement of the pump such that the output pressure of the pump does not exceed the maximum desired pressure level. However in actual practice, as the above-described control spring is compressed by the feedback mechanism, the force exerted on the control ring by the spring increases for further movement of the control ring toward the minimum displacement position. Thus the displacement of the pump is not decreased sufficiently to completely counter the increased operating speed and the output pressure of the pump continues to increase, albeit at a significantly reduced rate, in the regulated operating region.
It is desired to have a variable displacement vane pump which provides a substantially constant output, independent of operating speed increases, when the pump is in its regulated operating region.